Metamaterials are artificially engineered composites that can exhibit optical responses unattainable in their component materials. Indeed, electromagnetic metamaterials are composites engineered at the subwavelength scale to have specific optical properties. Behavior that is unattainable in any of the constituent materials can be demonstrated by carefully designing individual unit cells. The optical response of a metamaterial can be engineered by manipulation of the size, pattern, and composition of its subwavelength unit cells. This response, however, is usually fixed at the time of fabrication yielding materials that are essentially “passive” and operate over a limited bandwidth. The response of otherwise passive metamaterials can be rendered active by integrating dynamic components into the design. Active metamaterials control the resonant response of a material by incorporating dynamic components at the unit cell or substrate level. These active metamaterials, which seek to tune the resonant frequency range via control of the active medium, represent a new class of designs.
Introducing tunability by controllably activating a metamaterial system is important for the development of a number of devices including modulators, tunable filters and concentrators. Several approaches, ranging from electrical probing of single unit cells to thermal actuation, have been used to demonstrate amplitude modulation and frequency tuning of the resonant response. To date, amplitude modulation of optical metamaterials, which affects the intensity of the response at the resonant wavelength, has been achieved via electrical carrier injection in semiconductor substrates and mechanical reorientation of resonant elements using microelectromechanical systems. Frequency tunability has also been demonstrated by changing the dielectric environment of the resonator with phase-transition materials, liquid crystals, and optical pumping of the substrate.